Mantle upwelling, melt generation, and magma transport beneath mid-ocean ridges
Citable URI
https://hdl.handle.net/1912/5722Location
East Pacific RiseReykjanes Ridge
DOI
10.1575/1912/5722Abstract
The formation of new oceanic crust is the result of a complex geodynamic system in
which mantle rises beneath spreading centers and undergoes decompression melting. The
melt segregates from the matrix and is focused to the rise axis, where it is eventually
intruded and/or erupted to form the oceanic crust. This thesis combines surface
observations with laboratory studies and geodynamic modeling to study this crustal-production
system. Quantitative modeling of the crustal and mantle contributions to the
axial gravity and topography observed at the East Pacific Rise shows that the retained melt
fraction in the mantle is small (<3%) and is focused into a narrow column extending up to
70 km beneath the ridge axis. Consistent with geochemical constraints, the extraction of
melt from the mantle therefore appears to be efficiently focus melt toward the ridge axis. A
combination of laboratory and numerical studies are used to constrain the pattern of mantle
flow beneath highly-segmented ridges. Even when the buoyant component of mantle flow
is constrained to be two-dimensional, laboratory studies show that a segmented ridge will
drive three-dimensional mantle upwelling. However, using reasonable mantle parameters
in numerical models, it is difficult to induce large-amplitude three-dimensional mantle
upwelling at the relatively short wavelengths of individual segments (~50 km). Instead, a
simple model of three-dimensional melt migration shows that the observed segment-scale
variations in crustal thickness can be explained by focusing of melt as it upwells through a
more two-dimensional mantle flow field. At the Reykjanes Ridge, the melt appears to
accumulate in small crustal magma chambers, before erupting in small batches to form
numerous overlapping hummocky lava flows and small volcanoes. This suggests that
crustal accretion, particularly at slow-spreading centers, may be a highly discontinuous
process. Long-wavelength variations in crustal accretion may be dominated by variations in
mantle upwelling while short-wavelength, segment-scale variations are more likely
controlled by a complex three-dimensional processes of melt extraction and magma
eruption.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March 1997
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Suggested Citation
Thesis: Magde, Laura S., "Mantle upwelling, melt generation, and magma transport beneath mid-ocean ridges", 1997-03, DOI:10.1575/1912/5722, https://hdl.handle.net/1912/5722Related items
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